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Invasive earthworms threaten growth of new North American trees

An invading horde is spreading across the northern forests of North America, gobbling seeds and altering forest ecosystems as it goes. Who are these marauding horrors? Humble earthworms.

Despite their reputation as a gardener’s friend and contributor to soil fertility, these earthworms aren’t natives. The native earthworms were wiped out by glaciers during the last ice age, so the northern part of the US and Canada has probably been earthworm free for tens of thousands of years and every earthworm now living there is in fact an invader, usually from Europe.

The worms can cause dramatic changes to ecosystems by altering soils, reducing leaf litter and disrupting microbial interactions, which reduces biodiversity. Now it seems they are also eating plant seeds in the wild, potentially altering the make-up of forest communities.

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Hidden mouths

Ecologists Colin Cassin and Peter Kotanen of the University of Toronto in Mississauga, Canada, caged single earthworms with fine-mesh screen in 54 test plots in a mixed forest in Ontario, then sowed 15 seeds – marked with UV-fluorescent ink – into each plot. Two weeks later, they collected the soil in each plot and searched for seeds.

More than half the seeds of small-seeded species, but less than 15 per cent of those of larger-seeded plants, disappeared in worm cages. Because less than 2 per cent of seeds vanished from control cages that contained no worms, the missing seeds must have been eaten by the worms.

Cassin and Kotanen also used fiberglass-mesh screens to exclude either underground worms or above-ground rodents, or both, from forest plots with a natural store of seeds in the soil. After four weeks, plots with worms but without rodents had lost 47 per cent of their seeds, with small-seeded species again suffering the steepest losses.

Birch burden

The study shows another way that earthworms can alter forest ecosystems, particularly for small-seeded species such as birch, says Lee Frelich, an ecologist at the University of Minnesota in St Paul.

“We’ve always blamed lack of birch regeneration on other factors,” he says. “Now maybe we need to look at earthworms.”

Once earthworms have invaded a habitat, they are almost impossible to eradicate, says Erin Bayne, of the University of Alberta in Canada. Conservationists must instead work to keep worms out of pristine habitats, he says, for example by restricting the use of worms as fishing bait and by controlling accidental transport of contaminated soil.